For many purposes and particularly in association with automotive vehicles, it is desirable to provide an elongatable or contractable telescoping support having an inner bar received in an outer bar and displaceable relatively thereto to extend or contract the support. Such devices may be used, for example, as vehicle jacks or props, for support legs for semitrailers and in like applications. The principle, however, also has application wherever extendable supports are required, e.g. as construction jacks, spreading jacks or simply as elongatable structural members, pedestals and the like.
It is known to provide mechanical elongatable telescoping supports of the aforedescribed type and for the aforedescribed purpose which can comprise a support tube, a bar telescopingly received in the support tube and, at the mouth of the support tube from which the bar emerges, a so-called actuating head in which a drive roller is journaled.
The drive roller has a continuous surface which may be ribbed, milled or toothed for engagement with a running surface of the bar so that, upon rotation of this roller, the bar is extended out of the support tube. The bar may be formed with a complementary array of teeth or the like which mesh with the teeth or the roller in rack-and-pinion fashion.
The head can be provided with a guide surface for the roller so that, upon loading of the bar, actuation of the roller causes a reaction force which wedges the roller against the bar and effectively brings about a form-fitting engagement of the running surface of the bar and the roller which is guided toward the bar by the inclined running surface of the head.
In an extensible support of this type, the drive roller is held by its weight, i.e. by gravitational force, between the two converging running surfaces, i.e. is urged in the direction of convergence, so that elongation of the support by withdrawal of the bar from the support tube is facilitated while contraction of the support under load is resisted. To permit contraction of the support after the load has been removed, the roller must be lifted along the guide surface of the head.
A disadvantage of this system, however, has been found to reside in the fact that, upon rotation of the drive roller with a load applied to the bar, forces are generated which tend to tilt or cant the drive roller along the aforementioned running surfaces.
The teeth of the drive roller thus tend to bite into fresh surfaces of the running flank or face of the bar, i.e. tend to penetrate the bar in a manner which deviates from the orientation of the teeth of the latter or at locations other than where the teeth on the bar rack are provided, thereby reducing the reliability of the support and eventually damaging the rack surface of the bar.